The present disclosure relates to thermal transfer printers and particularly but not exclusively to methods for monitoring and controlling the quality of printed images.
Slip mode printing, as described in PCT WO97/36751 and later in PCT WO99/34983, is a known method of thermal transfer printing in which the printer controller controls the motion of the thermal transfer ribbon to be at a speed which is, to a chosen extent, less than the speed of the substrate to be printed on, whilst in the same process, controlling the signals to the thermal transfer printhead to print an image which is similarly reduced in size in the same plane as the direction of movement of the ribbon and substrate, so that as the thermal transfer prints, the ink is to some extent “smeared” onto the substrate. The desired result is that a full sized image is printed on the substrate, but the amount of ribbon consumed is less than the full size of the image, in the plane of the direction of movement of the ribbon and substrate.
There are two generally known modes of thermal transfer printing—continuous printing and intermittent printing. In both modes of printing, a printer performs a regularly repeated series of printing cycles, each cycle including a printing phase during which ink is being transferred to a substrate, and a further non-printing phase during which the apparatus is prepared for the printing phase of the next cycle.
In continuous printing, during the printing phase a stationary printhead is brought into contact with a printer ribbon the other side of which is in contact with a substrate on to which an image is to be printed. (The term “stationary” is used in the context of continuous printing to indicate that although the printhead will be moved into and out of contact with the ribbon, it will not move relative to the ribbon path in the direction in which ribbon is advanced along that path). Both the substrate and printer ribbon are transported past the printhead, generally but not necessarily at the same speed. Generally only relatively small lengths of the substrate which is transported past the printhead are to be printed upon and therefore to avoid gross wastage of ribbon it is necessary to reverse the direction of travel of the ribbon between printing operations to avoid ribbon wastage as is described in further detail below. Thus in a typical printing process in which the substrate is travelling at a constant velocity, the printhead is extended into contact with the ribbon only when the printhead is adjacent regions of the substrate to be printed. Immediately before extension of the printhead, the ribbon is accelerated up to a desired speed which may in normal operation be the speed of travel of the substrate. The ribbon speed is then maintained at the constant speed during the printing phase and, after the printing phase has been completed, the ribbon is decelerated and then driven in the reverse direction so that the used region of the ribbon is on the upstream side of the printhead. As the next region of the substrate to be printed approaches, the ribbon is then accelerated back up to the normal printing speed and the ribbon is positioned so that an unused portion of the ribbon close to the previously used region of the ribbon is located between the printhead and the substrate when the printhead is moved to the printing position. Thus very rapid acceleration and deceleration of the ribbon in both directions is desirable, and the ribbon drive system is ideally capable of accurately locating the ribbon so as to avoid a printing operation being conducted when a previously used portion of the ribbon is interposed between the printhead and the substrate.
In intermittent printing, a substrate is advanced past a printhead in a stepwise manner such that during the printing phase of each cycle the substrate and generally, but not necessarily, the ribbon, are stationary. Relative movement between the substrate, ribbon and printhead is achieved by displacing the printhead relative to the substrate and ribbon. Between the printing phase of successive cycles, the substrate is advanced so as to present the next region to be printed beneath the printhead and the ribbon is advanced so that an unused section of ribbon is located between the printhead and the substrate. Once again rapid and accurate transport of the ribbon is desirable to ensure that unused ribbon is always located between the substrate and printhead at a time that the printhead is advanced to conduct a printing operation.
Some commercially available thermal transfer printers are configured to operate in only one of intermittent and continuous modes. That is, the mode in which the printer operates is determined by constructional features of the printer. Other commercially available thermal transfer printers provide functionality such that a user can select either an intermittent mode of operation or a continuous mode of operation at runtime.